The present invention relates to an image forming apparatus, particularly to an image forming apparatus equipped with a fixing device.
Generally, in an electrophotographic image forming apparatus, a toner image based on image data is formed on a recording medium, and the recording medium with a toner image formed thereon is fed to a fixing device, where an unfixed toner image is fixed on the recording medium to get a printed image.
For example, the fixing device contains a heating roller as a heating member with a heater incorporated inside, and a pressure roller as a pressure member for forming a fixing nip through pressure contact with the heating roller. The heating roller and pressure roller apply heat and pressure while conveying a recording medium by sandwiching it with a fixing nip, and a toner image is fused on the recording medium.
The optimum fixing temperature differs according to the kind, weight and glossiness of the recording medium. For example, if the fixing temperature is too high, the coated paper whose surface is provided with special coating will be subjected to a blister where a blister-like defect appears on the surface. Conversely, if the fixing temperature is too low, fixing failure will occur to the paper of greater weight (as in thick paper). In color printing, the glossiness of the recording medium to be printed and outputted is adjusted by changing the fixing temperature in response to the glossiness specified by a user.
If the present temperature if the heating member is not the optimum fixing temperature, the image forming apparatus is put to the status of warm-up operation, and waits in that status until the present temperature of the heating member reaches the optimum temperature. To increase the temperature of the heating roller, the heater incorporated in the heating member is turned on. To decrease the temperature of the heating roller, the heater is turned off and the apparatus waits until the optimum temperature is reached by natural cooling.
The art of reducing the warm-up operation time is disclosed in the Patent Document 2, for example, wherein the target value of the fixing temperature is increased only when a recording medium made of special material such as thick paper or OHP sheet (overhead transparency film) has been specified, and the heating member and pressure member are rotated by pressure contact for a certain period of time during warm-up operation so that the pressure member is made warm, whereby the heat of the heating member is not deprived of by the pressure member when printing is started, and therefore, fixing failure is prevented.
[Patent Document 1] Official Gazette of Japanese Patent Tokkohei 7-58414
Incidentally, during the warm-up operation for reducing the temperature of the heating member, the pressure of the heating member and pressure member is released, and the apparatus is made to wait until the optimum temperature is reached by natural cooling.
However, when reducing the temperature of the heating member to the optimum temperature by natural cooling, much longer time is required than when increasing the temperature. During this time, printing cannot be started. Such a problem has remained unsolved in the prior art.
In view of the prior art described above, it is an object of the present invention to provide an image forming apparatus capable of reducing the warm-up operation time for reducing the temperature of the fixing member equipped with a heating means, down to the temperature lower than the present level.